The present invention relates to an optical fiber including an intermetallic compound to absorb hydrogen. The present invention also relates to an optical fiber cable including such an intermetallic compound for absorbing hydrogen.
An optical fiber itself comprises a light-guiding portion made of optionally-doped silica, and it is covered as a general rule by a primary covering and optionally by a secondary covering, generally made of polymer, in order to protect it from its environment. Such a covered fiber is usually intended for inclusion in a data transmission cable. An optical fiber cable used for transmitting data is a cable comprising at least one optical fiber included in a housing, which housing may be made in part out of metal or of plastics material, said cable possibly also having reinforcing components to provide traction strength or other metal components such as armoring or coverings.
The problem of attenuation in optical signal transmission due to hydrogen being absorbed into the silica lattice of optical fibers is known. Thus, U.S. Pat. No. 4,718,747, for example, proposes a solution which consists in introducing an element or an alloy of elements or an intermetallic compound of elements from groups III, IV, V, and VIII of the periodic table in an optical fiber structure or an optical fiber cable. Said elements are preferably lanthanides for group III, titanium, zirconium, and hafnium for group IV, vanadium, niobium, and tantalum for group V, and palladium for group VIII, and in still more preferred manner, these elements are lanthanum, zirconium, hafnium, vanadium, niobium, tantalum, or palladium. Palladium is the element that is in most widespread use in practice.
Unfortunately, such a solution implies using metals which are very expensive, for example the best known of them is palladium (Pd). Furthermore, the compounds obtained from groups III, IV, and V are easily polluted by other gases (such as carbon dioxide CO2, carbon monoxide CO, or oxygen O2) and can thus turn out to react very poorly with hydrogen once they have been polluted. Consequently, they are difficult to use, particularly industrially, since precautions need to be taken to ensure that they are conserved under an inert atmosphere, . . . . Finally, compounds based on elements from column VIII of the periodic table generally present high plateau pressures and form unstable hydrides with hydrogen. A demand has therefore appeared for compounds that provide better performance in terms of hydrogen absorption.
The intermetallic compound for absorbing hydrogen in accordance with the invention seeks to mitigate the drawbacks of prior art solutions, and in particular to enable hydrogen to be absorbed within optical fibers by forming stable hydrides of low plateau pressure, and thus greater capacity to reduce the presence of hydrogen within optical fiber cables.
The invention thus provides an optical fiber comprising a light-guiding portion and a peripheral portion around its light-guiding portion, the peripheral portion being constituted by at least one protective covering, the optical fiber including, in a covering outside its light-guiding portion, at least one crystalline intermetallic compound made up of at least two metals, the compound having a plateau pressure during hydride formation that is less than equal to 5xc3x9710xe2x88x922 atmospheres (atm) (where 1 atm=1.013xc3x97105 pascals (Pa)) measured by a PCT (pressure composition temperature) method and of the form ABxMy, where:
A is constituted by at least one element from columns IIa, IIIb, or IVb of the periodic classification of the elements (CAS version);
B is constituted by at least one element from columns Vb, VIII, or IIIa of the periodic classification of the elements (CAS version);
M contains at least one element from columns VIb, VIIb, Ib, or IIb of the periodic classification of the elements (CAS version);
with:
0xe2x89xa6xxe2x89xa610;
0xe2x89xa6yxe2x89xa63 if A contains elements from column IIa only; and
0.2xe2x89xa6yxe2x89xa63 if A contains at least one element from columns IIIb or IVb.
Said crystalline intermetallic compound present in a covering outside the light-guiding portion of the fiber can be incorporated in the optical fiber in various ways, for example by being incorporated in at least one covering of said fiber.
The invention also provides an optical fiber cable having at least one optical fiber, the cable including at least one crystalline intermetallic compound formed by at least two metals, having a plateau pressure during hydride formation that is less than or equal to 5xc3x9710xe2x88x922 atm measured at 30xc2x0 C. by a PCT method, the compound being of the form ABxMy, where:
A is constituted by at least one element from columns IIa, IIIb, or IVb of the periodic classification of the elements (CAS version);
B is constituted by at least one element from columns Vb, VIII, or IIIa of the periodic classification of the elements (CAS version);
M contains at least one element from columns VIb, VIIb, Ib, or IIb of the periodic classification of the elements (CAS version);
with:
0xe2x89xa6xxe2x89xa610;
0xe2x89xa6yxe2x89xa63 if A contains elements from column IIa only; and
0.2xe2x89xa6yxe2x89xa63 if A contains at least one element from columns IIIb or IVb.
In an embodiment of the invention, said crystalline intermetallic compound, whether present in an optical fiber or in an optical fiber cable is coated at least in part by a deposit of metal having the effect of protecting said intermetallic compound from pollution by a gas different from hydrogen such as oxygen O2, carbon monoxide CO, or carbon dioxide CO2, for example, and where said deposit nevertheless enables hydrogen to diffuse to the intermetallic compound. Such a metal deposit can be a deposit of nickel (Ni) or of copper (Cu).
Said crystalline intermetallic compound can be incorporated in the optical fiber cable in various ways, for example in a matrix, such as a DSM 3471-2-102 covering, in a polymer compound such as a Vestodur 3000 polybutylene terephthalate for the housing tube when said tube is made of polymer material, in a filler gel such as a Huber LA444 cable gel, or in any other manner available to the person skilled in the art.
Advantageously, such a crystalline intermetallic compound presents a plateau pressure which fixes the partial pressure of residual hydrogen in the cable at a very low level, less than or equal to 5xc3x9710xe2x88x922 atm so that attenuation is very low. One of the advantages of the crystalline intermetallic compound used in accordance with the invention in an optical fiber or in an optical fiber cable is that the equilibrium pressure plateau is preferably as flat as possible so as to maintain a performance level that is as constant as possible. Finally, another advantage of said crystalline intermetallic compound is that its absorption capacity is preferably as high as possible so as to maximize the lifetime of the optical fiber cable. This characteristic is generally expressed in terms of H/M, H/ABxMy, or as a percentage by weight.
Various examples of crystalline intermetallic compounds suitable for use in the invention in an optical fiber or an in optical fiber cable are given below.
A first example of a crystalline intermetallic compound usable in the invention is an alloy of the AB5 type, optionally at least partially, i.e. partially or totally, substituted by M. In the family of AB5 type alloys, the best known is LaNi5 which has a plateau pressure of 1.7 atm at 250xc2x0 C., which makes this compound unsuitable for the present invention. Substituting the Ni with Cr (column VIb) makes it possible to lower the plateau pressure to 0.04 atm for the compound LaNi4Cr when characterized at the same temperature; the resulting intermetallic compound thus comes within the invention. Substituting the Ni with Mn (column VIIb) makes it possible to lower the plateau pressure to 0.02 atm for the compound LaNi4Mn at 20xc2x0 C.; the resulting crystalline intermetallic compound thus comes within the invention.
A second example of a crystalline intermetallic compound suitable for use in the invention is an alloy of the AB2 type, optionally at least partially substituted by M. In the family of AB2 alloys, ZrFe1.4Cr0.6 has a plateau pressure of 3 atm at 20xc2x0 C., which means that this compound does not come within the invention. In contrast, total substitution of the Fe with Cr (column VIb) lowers the plateau pressure to 0.003 atm at 25xc2x0 C. for ZcCr2; the resulting crystalline intermetallic compound thus comes within the invention.
A third example of a crystalline intermetallic compound suitable for use in the invention is an alloy of the AB type, optionally at least partially substituted by M. In the family of AB type alloys, TiFe has a plateau pressure of 5.2 atm at 30xc2x0 C., which excludes this compound from the invention. In contrast, substituting the Fe with Cu (column Ib) makes it possible to obtain a plateau pressure of 0.00002 atm at 25xc2x0 C. for TiCu; the resulting crystalline intermetallic compound thus comes within the invention.
A fourth example of a crystalline intermetallic compound suitable for use in the invention is an alloy of the A2B type, optionally at least partially substituted by M. In the A2B family, compounds such as Zr2Cu or Ti2Cu have plateau pressures well below 0.1 atm at high temperatures (400xc2x0 C. to 600xc2x0 C.). It happens that plateau pressure decreases with temperature, which provides good possibilities for these compounds being usable in then context of the invention.
All of the crystalline intermetallic compounds belonging to the above-described families come within the invention. Nevertheless this list of compounds is not limiting. Other crystalline intermetallic compounds not belonging to the above-described families can also be proposed, such as Mg51Zn20 (MgZn0.39) which has a plateau pressure of 3xc3x9710xe2x88x927 atm at 25xc2x0 C. Similarly compounds of the ABxMy type can be envisaged in the present invention.
To summarize, the crystalline intermetallic compounds in the table below come within the invention:
The invention will be better understood and other characteristics and advantages thereof will appear on reading the following description given by way of non-limiting example and with reference to FIG. 1.
FIG. 1 is a simplified diagrammatic cross-section view of a unitube optical fiber cable. The cable 4 comprises an outer sheath constituted by a thermoplastic tube 3, and it contains optical fibers 1. The tube 3 is filled with a filler gel 2 which fills the space around the optical fibers 1 inside said tube 3.